Lithium-ion secondary batteries possess an electrode assembly wherein a separator is interposed between the positive electrode and the negative electrode. These lithium-ion secondary batteries can be charged and discharged by the movement of lithium ions between the positive electrode and negative electrode. A positive electrode active material that can reversibly achieve absorption and release of lithium ions is used in the positive electrode of this type of lithium-ion secondary battery. This positive electrode active material is composed primarily of an oxide containing lithium and a transition metal (lithium transition metal oxide). Using a solid solution formed by melting together a plurality of types of transition metal elements as a positive electrode active material has also been studied (for example, Patent Document 1 and Patent Document 2).